1. Field of the Invention
The present invention relates to a seek routine of a hard disk drive.
2. Background Information
Hard disk drives contain a plurality of magnetic heads that are coupled to rotating disks. The heads write and read information by magnetizing and sensing the magnetic fields of the disk surfaces. Each head is attached to a flexure arm to create a subassembly commonly referred to as a head gimbal assembly (“HGA”). The HGA's are suspended from an actuator arm. The actuator arm has a voice coil motor that can move the heads across the surfaces of the disks. The heads are typically electronically connected to the circuits of the drive by a flex circuit.
Information is typically stored in radial tracks that extend across the surface of each disk. Each track is typically divided into a number of segments or sectors. The voice coil motor and actuator arm can move the heads to different tracks of the disks.
FIG. 1 shows a typical track that has a number of fields associated with each sector. A sector may include an automatic gain control (“AGC”) field 1 that is used to adjust the strength of the read signal, a sync field 2 to establish a timing reference for the circuits of the drive, and ID 3 and Gray Code 4 fields to provide sector and track identification, respectively.
Each sector may have also a servo field 5 located adjacent to a data field 6. The servo field 5 contains a plurality of servo bits A, B, C and D that are read and utilized in a servo routine to position the head 7 relative to the track. By way of example, the servo routine may utilize the algorithm of ((A-B)-(C-D)) to create a position error signal (“PES”). The PES is used to create a drive signal for the voice coil motor to position the head on the track.
To move the heads to a new disk location the drive performs a seek routine by providing a control signal to the voice coil motor. In a seek routine the heads are moved across the disks and then decelerated into a settle mode. Disk drives typically include a control loop. The control loop includes a state estimator that computes head position and head velocity estimates that are used to adjust the control signal provided to the voice coil motor.
During the settle mode there may be a disturbance in the flex circuit that increases the settle time of the servo process. The disturbance may include a biasing force exerted onto the actuator arm by the flex circuit. There are various approaches for compensating the flex circuit disturbances. For example, the integrator state of the controller can be read during on-track mode. Another approach that utilizes a state predictor is disclosed in U.S. Pat. No. 7,394,614 issued to Kang and assigned to Samsung Electronics Corp., Ltd.
The biasing force of the flex circuit may change from a negative to a positive value. A negative bias tends to inhibit overshoot during a seek routine, but a positive bias can create a large overshoot, increase seek time and degrade the performance of the drive.